Many spinal surgical procedures involve the removal of a problematic intervertebral disc followed by the placement of a prosthetic implant (such as a fusion body, spacer or a motion disk) within the intervertebral disc space.
U.S. Pat. No. 6,083,228 (“Michelson”) discloses abrading elements particularly designed for preparing the intervertebral space in the spine for reception of the implant between adjacent vertebral bodies. FIG. 20 of Michelson discloses an abrading device having a pair of parallel abrading elements connected to a rotary drive mechanism which allows for simultaneous preparation of the adjacent endplates. However, the abrading surfaces of Michelson's devices preferably have a leading edge which removes material along that edge as it is advanced into the disk space. See, e.g., FIGS. 17A and 17B of Michelson. Accordingly, the anterior lips of the opposing vertebrae (which helps retain the implant in its desired space) are undesirably removed by the action of this device as the device enters the disc space. See FIG. 19 of Michelson.
When a variation in the height of the disk space must be accommodated, Michelson teaches using an adjustment screw in conjunction with a pair of wedge-shaped blocks, each having a threaded aperture for receiving the adjustment screw. According to Michelson, as the adjustment screw is turned, the wedged-shaped blocks move relative to one another to change the distance between the abrading surfaces. See col. 12, lines 45-57 of Michelson. The exact placement of these wedged-shaped blocks within the device is not explicitly described by Michelson. However, FIG. 20 also discloses a pair of opposed vertically-disposed frustoconical openings in the center of each abrading element 94 and 96. Accordingly, it appears that Michelson suggests that the wedged-shaped blocks are located in these frustoconcial openings, and that the adjustment screw is vertically-disposed therethrough. Since this vertically-disposed screw must be accessed from either the top or bottom of the device, it does not appear to be accessible to the surgeon once the abrading elements are inserted into the disc space. Accordingly, Michelson's “adjustable screw” device also appears to allow for a height adjustment only before the abrading elements are inserted into the disc space. Since these abrading elements are fixed prior to their entry into the disk space, it is reasonable to conclude that they also remove the anterior lips of the opposing vertebrae as this device is advanced into the disk space.
FIG. 23 of Michelson discloses a vibratory milling device whose abrading elements 218 have rounded working surfaces. Although it appears this device does not remove the anterior lips of the opposing vertebrae, since the device does not provide for in-situ height adjustment, the distance between these elements is fixed during operation, and so these elements must undesirably stretch the ligaments surrounding the disc space as they enter the disc space.
In sum, the devices of FIGS. 17 and 20 of Michelson undesirably eliminate the anterior vertebral lips which help retain the implant, while the device of FIG. 23 undesirably stretches the ligaments surrounding the vertebrae which help retain the implant.
Therefore, there is a need for a milling device which suitably prepare the disc space for the insertion of an implant, while retaining the anterior lips of the opposing vertebrae, but without stretching the vertebral ligaments
During endplate preparation, it is often desirable to increase the pressure upon the endplate being prepared in order to reduce preparation time. Michelson teaches one technique for increasing abrading pressure by using a device having an abrading surface on only one side of the device (as in FIG. 2 of Michelson) and using the non-abrading side as a fulcrum. However, the use of a non-abrading surface allows only for single-sided milling, and so doubles the time required for milling.
Michelson teaches sequentially selecting progressively larger abrading elements to form the desired disk space in a step-wise fashion (4, 45-46). The sequential nature of this activity makes for a time-consuming procedure.
In addition, Michelson does not teach measuring annular tension with the disclosed milling devices.
U.S. Pat. No. 5,865,846 (“Bryan”) teaches forming concave surfaces of predetermined shape in opposing vertebral endplates. However, the milling device used in such a procedure is not disclosed by Bryan.
U.S. Pat. No. 5,514,180 (“Heggeness”) discloses forming spinal prostheses having shapes which conform to the natural contour of the opposing vertebral endplates.
Therefore, there is a need for a milling device suitable for preparing a space in the human spine to receive an insert between adjacent vertebral bodies which can preserve the endplate lips without stretching the ligaments.